A conventional development apparatus of this type is disclosed in Patent Document 1, for example. The development apparatus is composed of a hopper 101 and a development section 102 as illustrated in FIG. 13. A toner 103 is held in the hopper 101, and is supplied through a supply outlet 105 to the development section 102 by rotation of a toner supply roller 104. A developer 106 in the development section 102 is a mixture of a magnetic carrier and a toner. The magnetic carrier and the toner are charged by friction as they are stirred by a stirring blade 107 (electric charge is provided to the magnetic carrier and the toner). A magnet roller 108 is composed of a rod-shaped magnet and a sleeve 108a. The magnet is fixed, and the sleeve 108a, which is made of a non-magnetic material (e.g., aluminum), is supported around the magnet in a manner which allows the sleeve 108a to freely rotate around the magnet. The developer is attracted by an outer circumferential surface of the rotating sleeve 108a due to the magnetic force of the magnet, and is transported by rotation of the sleeve 108a to a photosensitive body (not shown). A doctor blade 109 regulates a thickness of a developer layer on the outer circumferential surface of the sleeve 108a using an edge thereof.
When the toner in the developer layer on the outer circumferential surface of the sleeve 108a is charged by friction as the toner is stirred by the stirring blade 107, the charge of the toner has a polarity reverse to an electrostatic latent image on a surface of the photosensitive body, so that the toner is attached to the electrostatic latent image on the surface of the photosensitive body. Thereby, the electrostatic latent image on the surface of the photosensitive body becomes a visible image.
When a transport amount of the developer 106 is large, an excess amount thereof flows between a toner density sensor 110 and a bent portion 111a of a guide plate 111, slides down on the upper surface of the guide plate 111, and is returned to the stirring blade 107.
The toner density sensor 110 detects a toner density of the developer. As the toner of the developer is supplied to the photosensitive body, the toner density of the developer decreases. Therefore, the toner 103 is supplied from the hopper 101 to the development section 102 by the toner supply roller 104 so that the toner density detected by the toner density sensor 110 falls within a specified range.
However, even when actual measurement of the toner density is correct, the toner density of the developer is always inappropriate if there is an error in the specified range of the toner density, so that a faint image, a fog image, or the like occurs.
Therefore, for example, in Patent Document 2, the toner density is set so that Tn is 130 (%) or less, where Tn is a covering ratio of the toner to a surface of the magnetic carrier and the covering ratio Tn is defined by an expression below. In other words, a specified range of the toner density which causes the covering ratio Tn to be 130 (%) is set, and the toner density of the developer is caused to fall within the specified range.Tn=100C√3/{2π(100·C)·(1+r/R)2}·(r/R)·(ρt/ρc)]where r is a radius of the toner (μm), R is a radius of the magnetic carrier (μm), ρt is an absolute specific gravity of the toner (g/cm3), and ρc is an absolute specific gravity of the magnetic carrier (g/cm3).
Note that other patent documents also disclose a technique of setting a specified range of toner density using a diameter of a toner and a diameter of a magnetic carrier.
[Patent Document 1] JP H1-237577A
[Patent Document 2] JP H10-312105A